The present invention relates to a method of processing seismic data, in particular to processing seismic data to reduce the noise content of the data.
In a seismic survey, a source of seismic energy is actuated to emit seismic waves. The seismic waves propagate into and through the earth's interior, and are partially reflected by geological features within the earth. The reflected waves may be detected by suitable sensors, generally known as “receivers”. In a land-based survey, the receivers may be positioned on or below the earth's surface (for example within a borehole); in a marine seismic survey the receivers may be disposed on the sea-bed or may be disposed within a water column (as in a towed marine seismic survey). Information about the structure of the earth's interior may be derived from the seismic data acquired at the receivers following actuation of the seismic source.
One problem faced in processing seismic data is that the seismic data acquired at the receivers will contain, in addition to the desired signals caused by reflection of seismic energy by geological features within the earth, other signals that are not due to reflection of seismic energy by geological features within the earth. The signals that are not due to reflection of seismic energy by geological features within the earth are collectively referred to as “noise”.
Noise in seismic data has many causes. To give one example, in a towed marine seismic survey the seismic source and the seismic receivers are towed through the water by survey vessels. The propellers, and other machinery, of these vessels will generate acoustic energy, as will other vessels and drilling rigs near the survey location. Some of this acoustic energy will be transmitted into the water column and will be detected by the seismic receivers, so causing noise in the seismic data. Attenuation of the noise in acquired seismic data is a very important part of processing seismic data, and must be carried out without affecting the desired seismic signal.
Noise may be either coherent or incoherent, and coherent noise may be spatially coherent, temporally coherent, or both. Essentially, if noise is spatially coherent it is possible to make predictions about the noise at one location from knowledge of the noise at another location, and if noise is temporally coherent it is possible to make predictions about the noise at one time from knowledge of the noise at another time.
FIG. 1 illustrates the principle of a known method of attenuating coherent noise. FIG. 1 shows three seismic traces 1,2,3, acquired at respective spatially separated receivers. In FIG. 1, the vertical axis denotes time, and the horizontal axis denotes the offset of the trace (the “offset” of a trace is the source-to-receiver distance). Within each trace, the horizontal axis denotes the amplitude of the trace.
The traces contain a number of events. Two events are labelled: event A is assumed to be an event corresponding to reflection from a target geological feature, whereas event B is assumed to be noise. In trace 2 event A and event B occur at the same time, and the presence of event B makes it difficult to interpret event A correctly. Before trace 2 can be used to obtain information about the earth's interior it is necessary to attenuate or remove noise from the trace (or at least from the part of trace 2 around time tA, where tA is the time at which event A occurs in trace 2). The removal of noise from the trace is known as filtering the trace.
The method makes use of the fact that the variation in arrival time with offset—or “moveout”—for the desired event A is not the same as the moveout for the noise event B. This is indicated by the lines 4, 5 in FIG. 1 which show, respectively, the moveout of events A and B. The events A and B do not therefore occur at the same time in traces 1 and 3. It is therefore possible to derive a noise reference signal, which contains just event B, from trace 1 or 3. The reference signal may be derived from, for example, the region of trace 3 enclosed by the box 6. Since the noise event B is coherent between the traces, the noise reference signal derived from trace 1 or 3 should be a good estimate of the noise event B in trace 2. The noise reference signal may then be used to filter trace 2 (strictly, the reference signal derived from region 6 is used to filter a region of trace 2 around time tA).
The filtering process may be carried out using a multi-channel interference canceller 18 (or multichannel “filter”), as shown in FIG. 3(a). In its broadest term, an “interference canceller” is a device that receives as input a signal containing a desired component and an interfering component, and provides as an output a signal from which the interfering component has been cancelled (or substantially cancelled). The data trace that is to be filtered (in this case, trace 2), which is assumed to contain signal and coherent noise, is input into the primary channel 7 of the interference canceller 18. A reference signal derived from trace 1 or trace 3 is input into a reference channel 8 of the interference canceller. (In practice, the interference canceller 18 contains more than one reference channel, but only one reference channel 8 is shown in FIG. 3(a) for simplicity). The reference signal is a noise reference signal, and is essentially an estimate of the noise component of the data trace applied to the primary channel. The noise reference signal may be obtained as described above.
The interference canceller determines the parts of the data trace that are correlated with the noise reference signal. The correlated components are assumed to represent the noise in the data trace. An estimate of the signal component of the data trace is therefore obtained by subtracting the correlated components (which are taken to be an estimate of the noise in the data trace) from the data trace. This residual signal is then output from the output channel 9, as the estimate of the signal component of the data trace.
One interference canceller suitable for use with the method of FIGS. 1 and 3(a) is described in UK patent No. 2 309 082 and U.S. Pat. No. 5,971,095, the contents of which are hereby incorporated by reference. These disclose a multi-channel adaptive interference canceller, referred to as the ACONA adaptive interference canceller. The ACONA interference canceller has as its inputs a principal channel, to which the signal to be filtered is input, and one or more reference channels to which reference data are input. The interference canceller removes components in the signal input to the principal channel that are highly correlated with the reference data input to the reference channel(s), and outputs the residual signal that remains after removal of these components. The ACONA interference canceller is particularly effective at removing the noise component from the input data trace.
It is however difficult to apply the method of FIG. 1 if the moveout of the noise is similar to the moveout of the signal of interest. In this case, it is hard to derive a reliable noise reference signal.
A method of reformatting seismic data from common shot gathers to common receiver gathers to suppress noise is described for example in the European patent application EP-0201643. A method of transforming the balance of coherent events into into incoherent events in seismic data is described in the United Kingdom application GB-2237642 A.